1. Field of the Invention
This invention pertains to the general field of forming, forging and joining metals and alloys in a superplastic state in order to fabricate metallic structures. In particular, it provides a new way of isolating metals and alloys during the fabrication process from contaminants that affect the integrity and strength of the final product.
2. Description of the Related Art
It is known that certain metals and alloys exhibit the property of superplasticity when heated to a temperature within a given range below the phase transformation temperature. Under superplastic conditions, these metals and alloys display a high strain-rate sensitivity that permits high tensile elongations not otherwise achievable without affecting the strength of the metal or causing what is normally referred to as a "necking" effect, which is normally unacceptable. The metal can thus be formed in complex shapes by applying relatively low deformation stresses, which require less sophisticated equipment and result in less wear of the forming tools. The main problem with this process lies in the relatively high operating temperature, which promotes reaction with ambient contaminants that adversely affect the integrity of the metal. Therefore, it becomes necessary, at considerable additional cost, to use equipment that is capable of controlling the environment and ensuring the cleanliness of the metallic workpiece.
The process of diffusion bonding, by which metal surfaces are joined together through the commingling of atoms at the joint interface, is also enhanced by superplastic conditions. Inasmuch as pressure exerted for a period of time between adjoining surfaces of the same or different metals is the driving force for atomic diffusion between the two surfaces, the mobility of the molecules at a given pressure is greatly improved by the high temperatures required for superplasticity. As in the case of forming, the high operating temperatures promote the diffusion and reaction of ambient contaminants with the metal workpiece which embrittle the bond between the surfaces. Therefore, the process of diffusion bonding, which is often carried out concurrently with the forming of metals under superplastic conditions, also requires an environment that is as free as possible of contaminating substances.
Among the various metals utilized in the fabrication of metallic structures (such as titanium, hafnium, zirconium, and other metals), titanium and its alloys are known to exhibit superplastic properties greater than those of any other metal or alloy within a temperature range that is also suitable for diffusion bonding. Therefore, titanium and titanium alloys are particularly suitable for superplastic forming and diffusion bonding at temperatures ranging between 1,450.degree. F. and 1,850.degree. F. For a successful operation, though, it is necessary to heat and form the metal in a controlled environment that is as free as possible of oxygen, nitrogen and water vapor, which tend to form brittle oxygen, nitrogen and hydrogen compounds with the metal. Thus, several processes have been developed for protecting the surface of the titanium or titanium-alloy workpiece from coming into contact with these contaminants. Some methods describe the use of protective coatings to isolate the surface of the metal; others utilize inert atmospheres for carrying out the forming and diffusion bonding processes.
For example, U.S. Pat. No. 2,903,785 to Hanink et al. (1959) discloses a method of forging titanium by first coating it with a titanium-aluminum alloy. The coating is formed on the surface of the workpiece by immersing it in a fused salt bath that provides a protective layer to avoid oxidation of the titanium at the high temperatures required for forging. The workpiece is then immediately immersed in a molten aluminum-coating metal, wherein the aluminum forms an alloy with the titanium on the surface of the piece. After forging, the superficial alloy is removed from the finished product.
In U.S. Pat. No. 3,920,175 (1975) and U.S. Pat. No. 3,927,817 (1975), Hamilton et al. describe a method for fabricating metallic structures by superplastic forming and diffusion bonding under an inert gas and/or a vacuum environment in order to prevent contamination of the metal surfaces. Fluid pressure loading is utilized to cause the workpiece to deform against a shaping structure under superplastic conditions.
Both approaches have some drawbacks in spite of the improved operating conditions resulting therefrom. The process of coating the surface of a blank workpiece before forming it at superplastic temperatures entails several steps that may not be otherwise necessary or desirable. The protective coating must first be deposited on the piece under relatively clean conditions and then it must be removed to obtain the desired end product. Similarly, the approach of conducting superplastic forming under inert conditions is difficult to achieve because some contamination often occurs as a result of small quantities of oxygen and other contaminants in the inert gas. Thus, all efforts to provide some protective insulation for these metals during deformation have proven to be relatively unsuccessful. In addition, all current processes are time consuming because many steps have to be taken (such as coating the workpiece with a protective material; producing an inert environment; and creating a vacuum) that are not directly contributing to the forming process. These processes also require expensive equipment, such as needed for maintaining a sealed environment under vacuum and for providing the required heat and pressure differential for forming the workpiece in an entirely self-contained process chamber.
Another approach is based on the idea of forming an oversize structure at superplastic temperatures with minimal contamination controls, permitting the outer layer of the product to be embrittled by contamination. This layer is then machined off, obviously at considerable expense and waste of material.
Therefore, any process, material, or technique that might result in the creation of a contaminant-free environment under the operating conditions required for superplastic forming and diffusion bonding would be of great usefulness and commercial value to the industry. The present invention deals with a new approach to providing such an environment while retaining all conditions that are necessary for a successful process. That is, the method of this invention is practiced at the proper temperature range for allowing workpiece deformation without fracture; it is not subjected to time limitations, so that the correct strain rate can be applied to the workpiece; and it is suitable for different working pressure to effect either deformation or bonding, or both, as required by the particular application.